1. Field of the Invention
This invention relates to a speech recognition LSI system, and more particularly to an A/D output processing unit for converting a speech input signal into a digital signal and then sampling-processing the digital signal
2. Description of the Related Art
In a conventional speech recognition LSI system, a speech input signal is subjected to various speech processings--for example, rectification for respective frequency bands, selection of a low frequency band, and analog/digital conversion--and the result of a given processing is sampled at a constant frame period or interval as an acoustic characteristic parameter. The processing circuit for performing the above speech processings includes a plurality of band-pass filters having different frequency bands, rectifier circuits for rectifying outputs from the respective band-pass filters, low-pass filters for smoothing outputs of the respective rectifier circuits, a switching circuit for selecting outputs from the respective low-pass filters, an A/D converter for analog-digital (A/D) converting the output selected by the switching circuit, and a sampling circuit for sampling an output from the A/D converter.
When an output of the A/D converter is sampled at a frame interval (T) of, for example, 20 msec, it is then necessary to set the cut-off frequency (fc) of the low-pass filter at the preceding stage at less than 25 Hz (1/2 T). Otherwise, as indicated by the sampling theory, aliasing noise is added to sampled data. However, it is difficult to form the low-pass filter having cut-off frequency fc of 25 Hz in the LSI because it is necessary to form a capacitor with a large capacitance in order to form the low-pass filter having a low cut-off frequency. The capacitor with a large capacitance occupies a large area in the LSI. Further, it is necessary to form an insulation film with a thin and constant thickness as a capacitor insulation film, making the manufacturing process complex. For this reason, in the practical speech recognition LSI, cut-off frequency fc of the low-pass filter is set at approximately 50 Hz and the sampling frame interval is set shorter than 10 msec (1/2.times.1/fc) in order to enhance the reproductivity of the speech input signal.
However, if the sampling interval is set shorter than 1/2 T, sampling data of a quantity larger than twice that obtained when the cut-off frequency is set at T can be obtained for the speech input signal of the same speech generation period of time. Therefore, the operation efficiency of a memory which is used for storing the sampling data and which is provided in a recognition processing circuit for checking the matching between the sampled data and reference patterns may be lowered, and the processing time for speech recognition may be increased.